In general, wrapping materials must have various characteristics depending on factors such as the type, form and weight of the goods to be wrapped. It is particularly necessary for wrapping materials for making bags to have good physical strength, such as tear strength, tensile strength, burst strength, and impact strength. It is also desirable if wrapping materials used in making bags not to retain a static electrical charge.
U.S. Pat. Nos. 4,147,291 and 4,258,848 disclose wrapping materials having improved physical strength. The layer construction of these wrapping materials is shown in FIG. 1.
FIG. 1 shows a known wrapping material comprised of two mono-axially stretched, high density polyethylene films 1 and 2, placed in such a manner that the stretching axis of the film 1 forms an angle of 45.degree. to 90.degree. with that of the film 2. An adhesive layer 3 is sandwiched between films 1 and 2. This is a so-called cross-laminate film wrapping material. Although the adhesive layer 3 made of low density polyethylene is used in this wrapping material, it is unnecessary to always provide such an adhesive layer. The films 1 and 2 may be heat-sealed at an appropriate temperature of about 180.degree. C.
The cross-laminate film wrapping material has a high degree of strength in the longitudinal, lateral, and oblique directions because the stretching axes of the films cross each other. Accordingly, the physical strength is markedly improved at least compared with a conventional composite laminate film wrapping material composed, for example, of a low density polyethylene film and paper, an aluminum foil, or the like.
The flexibility and physical strength of the cross-laminate film wrapping material will vary with the thickness of the adhesive layer 3. That is, it has been found that as the thickness of the adhesive layer 3 is increased, the physical strength is reduced and the flexibility is deteriorated.
If the mono-axially stretched thermoplastic resin film contains no antistatic agent or only a small amount of antistatic agent, it generates a large quantity of static electricity when leaving a pass roll, in the course of high-speed winding or high-speed bag making, thereby causing operational problems.
To overcome such operational problems, a method is used in which an aluminum foil is laminated, or a method is used in which the production speed is lowered and a large amount of carbon black is added to the thermoplastic resin film. The utilization of such a method makes it possible to obtain a light-shield preventing effect and an antistatic effect at the same time. However, such methods are economically unsatisfactory.